Enterobacteria are facultative aerobes, and when cultured in the absence of oxygen can use a variety of alternative respiratory oxidants. Nitrate and nitrite are the preferred alternatives, and respiratory enzyme gene expression is tightly controlled in response to their availability. This control is mediated by dual interacting two-component regulatory systems. Nitrate and nitrite control the autophosphorylation of two cytoplasmic membrane sensor-kinases, NarX and NarQ, which then phosphorylate two response regulators, NarL and NarP. The phosphorylated response regulators bind to DNA in target operon control regions. Thus, transcription initiation is activated or repressed according to nitrate and nitrite availability. Homologous two-component systems control a variety of other processes in diverse bacterial species. The Nar regulatory system exhibits several complexities that are amenable to experimental investigation, including (1) differential recognition of structurally-related ligands; (2) differential sensor-regulator interactions that influence both kinase and phosphatase reactions; (3) differential recognition of DNA binding site arrangement; and (4) differential transcription activation at different target control regions. Proposed experiments will employ both in vivo and in vitro approaches to probe specificity determinants that control these differences. Results will broaden our understanding of two-component signal transduction. They will also strengthen our appreciation for the complexities of enterobacterial anaerobic physiology, which is relevant to understanding the physiology of enterobacterial pathogens such as enteropathogenic E. coli, Shigella spp. and Salmonella spp. in their anaerobic habitat, the mammalian intestine.